Optical communication systems employ a variety of voltage-driven devices to generate light modulation. One of the most commonly used devices is the Mach-Zehnder interferometric modulator (MZM). The MZM has a wavelength-independent transfer function; besides, it can be operated in a dual electrode structure, that—in comparison to a single electrode modulator—requires lower driving voltages and provides chirp-free optical output.
For a SiPh MZM, a reverse-biased junction is typically used to generate the electro-optical effect. One drawback of this approach is that a reverse-biased junction has very small capacitance and therefore a long modulator with relatively high driving voltage and losses is needed to achieve the required extinction ratio. Travelling wave electrode or distributed amplifiers are the most common driving schemes.
To reduce the size and the losses of the modulator, a forward-biased junction is to be preferred. A forward-biased junction works as an injection device and—as such—it has very large capacitance (>1 pF) to create enough extinction ratio, given its short length. In order to drive this heavy load, a low impedance driver is required. The best driver would consist of a B-class output stage; also known as push-pull. In a CMOS technology this implementation is straightforward, but in bipolar technologies, where usually only lateral PNP transistors are available, the most suitable stage is the emitter follower (EF).
In an EF, when the output goes high not all the current from the top transistor is available to charge the load, because some of the current is lost by flowing through the bias transistor, thus slowing down the transition time.
One way to circumvent this shortcoming is to provide an auxiliary path for the signal to activate the bias transistors during the transitions, turning it into active pull-down element, that sinks or sources an extra amount of current from or into the output node, thus speeding up the transitions.
This principle has been described in U.S. Pat. No. 6,707,589 B2 and employed for a single-ended EAM driver. The same principle is also applied for a differential MZM driver in a paper by Enrico Temporiti et al. entitled “Insights Into Silicon Photonics Mach-Zehnder-Based Optical Transmitter Architectures” IEEE Journal of Solid-State Circuits (Vol. 51, No. 12, pp. 3178-3191, December 2016).
One shortcoming of the foregoing implementations is that the load capacitance seen by the predriver stage (hereinafter also referred to as preamplifier) is increased by the series capacitance of the coupling capacitor and the input capacitance of the pull-down device, therefore it is not possible to improve the output transition times without impairing the bandwidth of the predriver. This is especially true when low power and high voltage swings are required.
It is possible to include additional buffers between the predriver and the output stage, but this would cause an increase in the overall current consumption. Besides, as shown in U.S. Pat. No. 6,707,589 B2, the supplementary buffers demand a higher supply voltage to prevent the saturation of the pull-down element, which turns into extra power consumption.